Preheat oven to 350°F (175°C). Spray a 9x5-inch loaf pan with nonstick cooking spray (or grease with butter). Set aside.
Whisk together flour, baking powder, cardamom, ginger, and salt in a large mixing bowl until thoroughly combined.
In a separate medium-sized bowl, whisk together melted butter and granulated and brown sugars.
Whisk sour cream, eggs, lemon zest, 1 tablespoon lemon juice, and vanilla extract into butter-sugar mixture.
Add butter mixture to flour mixture. Use a spatula to gently fold ingredients together, stopping when mostly combined (spots of flour should still be visible).
Fold rhubarb and nuts into batter just until combined (do not over-mix).
Spread batter in prepared loaf pan, lightly smoothing top.
Sprinkle top of loaf with demerara sugar.
Bake until well-browned and a toothpick inserted in center (and a few other spots) comes out clean, about 50 to 60 minutes.
Let loaf cool completely before slicing. Enjoy!
Notes
When measuring the flour, use a spoon to add it to your measuring cup, then level off the top with a straight edge (such as a knife). Avoid scooping the measuring cup directly into the flour ... this compresses the flour, causing you to use too much.I used light brown sugar, but dark brown sugar would also work.Demerara sugar is a coarse brown sugar with large crystals. It gives the top of the bread a delicious, sweet crunch. Turbinado sugar would also work great. Feel free to skip the sugar if you can't find it.This bread is really rich and dense. Beware of under-baking it. Your loaf should be well-browned when done. Make sure to use a toothpick to test it.Check out the info before this recipe card for helpful tips, step by step photos, variations, related recipes, and more!